The present invention relates to ultra fine particles of Tungsten(W) and a method for producing the same.
Metal particles show properties different from that of ordinary ones (of 1 xcexcm or more, for instance) when a diameter becomes ultra fine such as 100 nm or less. In the ultra fine particle, since a number of atoms on the surface thereof increases relative to the total number of atoms, an influence of surface energy on properties of the particle can not be neglected. Further, an influence due to residual strain that is a problem in an ordinary bulk material can be avoided. Thereby, the ultra fine particles show various kinds of excellent properties.
The ultra fine particles show properties different from that of a bulk such as that for instance a melting point and sintering temperature thereof become lower compared with that of the bulk, and, in some cases, hardness becomes higher compared with that of the bulk. Further, when a plurality of ultra fine particles exists, there is a likelihood that a tunneling effect occurs therebetween, or a quantum mechanical effect such as quantum well, mini-band, quantum wire or the like appears. Further, the ultra fine particles, depending on the type thereof, have an excellent function of various kinds of materials such as obtaining high catalytic activity. Thus, the ultra fine particles are excellent in various kinds of properties such as chemical, mechanical, electrical, thermal properties and so on.
By making the best use of the excellent properties that the ultra fine particles of nano-order have, various kinds of materials can be improved in properties thereof and various kinds of devices and catalysts can be applied to functional materials. Accordingly, the study of physical properties and applications of the ultra fine particles is in progress. Further, if an agglomerate thereof maintaining characteristics of the ultra fine particle of nano-order, for instance, a nano-crystal thin film can be obtained, a likelihood of applications as the device material and functional material is expected to become even higher.
Now, as existing methods of producing the ultra fine particles, there are known physical and chemical methods shown in the following. That is, as the physical methods for producing the ultra fine particles, a gas phase condensation method, a sputtering method, a metal evaporation synthesis method, a vacuum evaporation method on a fluid oil can be cited. As the chemical methods for producing the ultra fine particles that make use of a liquid phase, a colloidal method, an alkoxide method, a co-precipitation method or the like can be cited. As the chemical methods for producing the ultra fine particles that make use of a gaseous phase, an organometallic compound pyrolysis method, a metal chloride reducing/nitridation method, a reduction method in hydrogen, a solvent evaporation method can be cited.
All the aforementioned existing methods for producing the ultra fine particles are ones that obtain the ultra fine particles as agglomerates, that is, ultra fine powders. Accordingly, these methods are unsuitable for studying properties and applications as a single ultra fine particle.
By contrast, inventors of the present invention previously proposed a method of generating Al ultra fine particles in which an electron beam of an intensity of approximately 1xc3x971020 e/cm2xc2x7sec is irradiated on an Al oxide particle in an atmosphere of a high vacuum to generate Al ultra fine particles (cf. Japanese Patent Laid-open Application No. HEI 8-217419). According to the method, the Al ultra fine particles can be obtained as a single particle, further the shape and crystal orientation thereof can be controlled.
However, since the aforementioned method has been developed only for the Al ultra fine particles, conditions of production of the ultra fine particles are not necessarily suitable for all kinds of metals.
For instance, W is known as a refractory metal. If ultra fine particles of W satisfying both such material properties of W and properties based on the ultra fine particles can be obtained, applications to usage different from the Al ultra fine particles can be expected. However, Al pertaining to light metals and W that is one of elements of particularly large atomic weight among heavy metals show largely different behavior in irradiating an electron beam on oxide particles thereof. Accordingly, even if the aforementioned conditions for producing the Al ultra fine particles are simply applied to the W ultra fine particles, excellent W ultra fine particles can not be obtained with good reproducibility. This hinders the W ultra fine particles from being applied to devices and various kinds of function materials. From the above, development of conditions for producing ultra fine particles that enable to obtain W ultra fine particles with reproducibility is in demand.
In addition, if an agglomerate maintaining properties of ultra fine particles of nano-order, for instance, a nano-crystal thin film is obtained, realization of applications in for instance device materials and function materials is highly expected to be further heightened. From the above, it is in demand to enable production of a nano-crystal thin film that uses W ultra fine particles.
There are existing general thin film formation methods such as a PVD method and CVD method typical in a vacuum deposition method, a laser ablation method, a sputtering method or the like. Furthermore, there are modified methods of the above ones such as a molecular beam epitaxy method (MBE method), a metal-organic vapor phase epitaxy method (MO-VPE method) or the like, in all of which controllability of the aforementioned methods are improved. In these general thin film formation methods, due to single-crystallization of the film caused by a substrate for film formation and non-uniformity in the initial stage of film formation, further due to crystal growth caused by heating of the substrate, it is extremely difficult to uniformly control the crystal size in nano-order.
The object of the present invention is to provide ultra fine particles of W capable of being operated and controlled in various ways as a single particle or a fused body, and a method for producing ultra fine particles of W that enable to obtain such ultra fine particles of W with good reproducibility.
The present inventors studied hard to obtain excellent ultra fine particles of W. As a result, it is found that in irradiating an electron beam on a particle of W oxide to produce ultra fine particles of W, the electron beam of an intensity in the range of 1023 to 1024 e/cm2xc2x7sec is effective.
That is, in the case of an electron beam being irradiated on a particle of WO3 or so on, when the intensity of the electron beam is less than 1023 e/cm2xc2x7sec, energy is insufficient for debonding between W and oxygen atoms. In this case, only the inside of the particle of WO3 is changed to a fine polycrystalline structure, ultra fine particles of W being not obtained from the particle of WO3.
On the other hand, in the case of the intensity of the electron beam exceeding 1024 e/cm2xc2x7sec, the particle of WO3 is damaged due to the irradiation, ultra fine particles of W of excellent crystalline state being not obtained. It is considered that on these, an atomic weight of W and bonding energy between W and oxygen affect. In particular, the atomic weight is considered to largely affect thereon.
The present invention is based on such knowledge. Ultra fine particles of W of the present invention are ones that are formed through an irradiation of an electron beam on a particle of W oxide. It is characterized in that the ultra fine particles consist essentially of W that is derived from the particle of W oxide by irradiating the electron beam of an intensity of 1023 to 1024 e/cm2xc2x7sec on the particle of W oxide in an atmosphere of a high vacuum.
The ultra fine particles of W of the present invention have a particle diameter of for instance a diameter of 10 nm or less. The ultra fine particles of the present invention can exist not only as a single particle of ultra fine particle of W but also as a bonded body of a plurality of ultra fine particles of W. As a concrete example of this case, a nano crystal thin film in which a plurality of ultra fine particles of W is bonded can be cited.
A method for producing ultra fine particles of W of the present invention comprises a step of disposing a particle of W oxide on an amorphous carbon film, and a step of irradiating an electron beam of an intensity of 1023 to 1024 e/cm2xc2x7sec on the particle of W oxide in an atmosphere of a high vacuum to debond W atom from the particle of W oxide to form ultra fine particles of W.
In the present method for producing ultra fine particles of W, the ultra fine particles of W derived from the particle of W oxide, while sticking on the amorphous carbon film, can be bonded to each other. In the method of producing ultra fine particles of W of the present invention, an electron beam is preferable to be irradiated on a particle of W oxide in an atmosphere of 10xe2x88x925 Pa or better.